The present invention relates to a liquid solution optical filter which is transparent to the visible, and absorbs the near infrared radiation. The invention is particularly useful for optically filtering the solar radiation received by greenhouses, in order to shield the interior of the greenhouse from the infrared heat energy in such radiation, and to store the absorbed heat. The invention is therefore described below with respect to this application.
About half of the solar energy that reaches the surface of the earth is in the visible region and is utilized by plants for photosynthesis. The remaining half of the solar energy is in the near infrared region and has no function except heating. There are many applications of optical filter solutions which absorb the heat irradiation in the near infrared region while transmitting therethrough the radiation in the visible region. An important application in greenhouses, where the optical filter solution not only reduces the heat penetrating into the interior of the greenhouse during the daytime, but also stores the heat and uses same for heating the greenhouse interior during the nightime.
U.S. Pat. No. 4,108,373, discloses one such application for greenhouses. The optical filter described in that patent is an aqueous solution of copper chloride in water in a proportion of 25 grams/liter. However, it has been found that the solution suggested in this patent does not have good spectral properties, since it exhibits strong absorption in the visible region.